Steam generating systems frequently produce superheated steam and deliver that steam to utilization devices such as steam turbines or the like. Because superheated steam can reach temperatures which damage the utilizing devices or the superheaters itself, close control is maintained over the superheat temperature of the steam. There are several known techniques for controlling superheat temperature and desuperheating the steam where necessary, and one such technique is reducing the superheat temperature by injecting a water spray into the steam. This water spray reduces the superheat temperature by the amount of heat required to raise the injected mass of water to the vaporization temperature and then to vaporize the water.
A relatively close degree of control for superheat temperatures is desired in many applications. An effective desuperheat apparatus should modulate the flow of water or other desuperheat liquid over a range from the desired maximum flow down to a minimum flow rate. Because efficient desuperheating use of the injected water is maximized by reducing the size of water droplets and maintaining a desired spray dispersion pattern of those droplets into the steam flow, an effective desuperheat apparatus should maintain the desired spray dispersion and water droplet size over the full range of fluid flow rates. A spray dispersion pattern in the shape of a hollow cone spray has been found particularly effective for steam desuperheat applications.
Spray nozzles having a fixed geometry require some minimum pressure drop across the spray orifice to maintain the desired spray pattern. This minimum pressure drop is easily maintained at higher liquid flow rates, namely, flow rates at least sufficient to maintain a back pressure behind the nozzle orifice. However, as the desuperheat valve throttles the liquid flow rate below some particular minimum flow rate, the back pressure is no longer maintained and the pressure drop across the nozzle decreases. Consequently, the spray dispersion pattern becomes degraded and the individual water droplets making up the spray may become enlarged at relatively low rates of flow. These effects, in turn, reduce the efficiency of desuperheating at relatively low rates of water flow, because the water spray contains fewer droplets per unit volume and thus has a correspondingly reduced surface area, per unit volume of water, for receiving heat transfer from the superheated steam.
Prior art attempts to overcome the foregoing problems have generally used multiple spray nozzles, with one nozzle producing the desired spray dispersion pattern at relatively high rates of water flow and with another nozzle designed to produce an efficient spray pattern at the relatively low rates of flow. These multiple nozzle desuperheaters are mechanically more complex, requiring an operating mechanism coordinating and controlling the liquid flow to both nozzles, and also controlling the geometry of the one or both nozzles in some cases. Furthermore, it has been found that multiple nozzle desuperheaters sometimes produce overlapping sprays; these spray patterns impinge each other, producing larger water droplets which negate the purpose of a separate low-volume spray nozzle.
It is an object of the present invention to provide an improved spray desuperheat apparatus.
Another object of the present invention is to provide a spray desuperheat apparatus providing a substantially uniform spray dispersion pattern over a range of flows including relatively low flow rates.
It is still another object of the present invention to provide a spray desuperheat apparatus providing spin control of the spray pattern at relatively low flow rates, and providing pressure control of the flow pattern at relatively higher rates of flow.
The foregoing and other objects and advantages of the present invention will become more readily apparent below.